Unexpected token token in reducerName – How to solve this Elasticsearch exception

Opster Team

August-23, Version: 6.8-8.9

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Briefly, this error occurs when Elasticsearch encounters an unexpected token in the reducer name. This could be due to a syntax error, incorrect reducer name, or an unsupported character in the reducer name. To resolve this issue, you can: 1) Check the reducer name for any syntax errors and correct them. 2) Ensure that the reducer name is correctly spelled and matches the one defined in your Elasticsearch configuration. 3) Remove any unsupported characters from the reducer name.

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This guide will help you check for common problems that cause the log ” Unexpected token ” + token + ” in [” + reducerName + “]. ” to appear. To understand the issues related to this log, read the explanation below about the following Elasticsearch concepts: aggregations.

Log Context

Log “Unexpected token ” + token + ” in [” + reducerName + “].” class name is BucketSelectorPipelineAggregationBuilder.java. We extracted the following from Elasticsearch source code for those seeking an in-depth context :

 "Unknown key for a " + token + " in [" + reducerName + "]: [" + currentFieldName + "]."
 } else {
 throw new ParsingException(parser.getTokenLocation(); "Unexpected token " + token + " in [" + reducerName + "].");
 }  if (bucketsPathsMap == null) {
 throw new ParsingException(


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